The present invention relates to an optical semiconductor device and a method for fabricating the optical semiconductor device, more specifically, an optical semiconductor device having the detection sensitivity increased and a method for fabricating the optical semiconductor device.
Infrared sensors of the quantum well type which are applicable to detection of two wavelengths of infrared radiation.
The conventional infrared sensor, i.e., the conventional optical semiconductor device will be explained with reference to FIGS. 19A and 19B. FIG. 19B is a plan view of one picture element of the conventional optical semiconductor device, which is on the side of the substrate of the sensor device. FIG. 19A is a sectional view along the line A-Axe2x80x2 in FIG. 19B.
As shown in FIG. 19A, on the side of the substrate 116, which is on the side of the sensor device substrate a picture element isolation insulation layer 118 is formed. A contact layer 120 is formed on the picture element isolation insulation layer 118. An MQW (Multi Quantum Well) layer 126 is formed on the contact layer 120. A contact layer 128 is formed on the MQW layer 126.
On the contact layer 128, an MQW layer 134 having different light absorbing characteristics from the MQW layer 126 is formed. A contact layer 136 is formed on the MQW layer 134.
An insulation film 140 is formed on the contact layer 136. An optical coupling layer 144 is formed in stripes on the insulation film 140.
A mirror electrode 164 is formed on the upper surface and the side surface of the optical coupling layer 144 formed in stripes. The mirror electrode 164 and the optical coupling layer 144 make up an optical coupler. Light incident on the side of the substrate 116 is scattered by the optical coupler to be absorbed by the MQW layers 126, 134.
In such optical semiconductor device, an opening 200 and an opening 202 are formed from the upper surface of the mirror electrode 164 respectively down to the contact layer 120 and the contact layer 128. The openings 200, 202 are diverged gradually from the contact layers 120, 128 to the mirror layer 164. A picture element isolation groove 166 for isolating each picture element is formed down to the picture element isolation insulation film 118. An insulation film 168 is formed on the entire surface.
Contact holes are further formed in the bottoms of the openings 200, 202, arriving at the contact layers 120, 128. Ohmic electrodes 162c, 162a are formed respectively on the contact layers 120, 128 in the contact holes.
As shown in FIG. 19B, three connection electrodes 114a, 114b, 114c are formed in a cylindrical shape outside the openings 200, 202. The connection electrodes 114a, 114b, 114c are to be connected to a reading circuit substrate (not shown) for reading detected signals.
The connection electrode 114a is connected by a line 206 to the ohmic electrode 162a formed on the contact layer 128. The connection electrode 114c is connected by a line 204 to the ohmic electrode 162c formed on the contact layer 120. The connection electrode 114b is connected to an ohmic electrode 162b formed on the contact layer 136.
The mirror electrode 164 is formed on the entire surface, interrupted around the connection electrodes 114a, 114b, 114c for the prevention of short-circuit among the connection electrodes 114a, 114b, 114c. 
In such optical semiconductor device, a direct-current bias is applied from the side of the reading circuit substrate via the connection electrode 114a. An output of the MQW layer 134 is supplied to the reading circuit substrate via the connection electrode 114b. An output of the MQW layer 126 is supplied to the reading circuit substrate via the connection electrode 114c. 
However, in the conventional optical semiconductor device, the openings 200, 202 are formed large, and especially the opening 200 arriving at the lower MQW layer 126 is made larger, which makes an area of the optical coupler and an area of the MQW layers 126, 134 accordingly small. That is, a light-receptive area of the optical semiconductor device of such structure is too small to obtain sufficient sensitivity.
An object of the present invention is to provide an optical semiconductor device having improved optical detection sensitivity and a method for fabricating the optical semiconductor device.
The above-described object is achieved by an optical semiconductor device comprising: a first contact layer formed on a substrate; a first quantum well layer formed on the first contact layer; a second contact layer formed on the first quantum well layer; an optical coupling layer formed on the second contact layer; and a first conductor plug extended from an upper surface of the optical coupling layer and arriving at the first contact layer. The contact layers and the connection electrodes are connected to each other respectively by the conductor plugs. The conductor plugs have such small sectional areas that the quantum well layers can secure sufficiently large areas. Accordingly, high sensitivity can be provided. Upper portions of the conductor plugs are also etched in stripes, and can function as an optical coupler, whereby light can be scattered, with a result of increased sensitivity. The conductor plugs are buried in the contact holes, whereby the connection electrodes can be formed on the conductor plugs. Accordingly, higher freedom of design can be obtained in arranging the connection electrodes.